In electric resistance welding or spot welding it is often required to use a fixture to hold sheet metal workpieces in the correct orientation for assembly. Generally such fixtures have a body made of a strong material which is heat conductive such as copper or steel and support a fixed copper electrode in contact with the workpiece. Movable electrodes engage the workpieces opposite the fixed electrode and apply welding current which passes from one movable electrode through the sheet metal to the fixed electrode and back through the sheet metal to the other movable electrode to form the welds. In some cases, the fixed electrode is coupled to the weld current source and the current flows in only one direction through the workpiece. The fixed electrode in either case is thus subject to heat from the welded workpiece and is also directly heated by the current passing through it. To dissipate the heat it is important that the fixture act as a heat sink and it may be water cooled. It is also important that the electrode be electrically insulated from the fixture body. A separate insulator between the electrode and fixture body provides electrical insulation but reduces electrode cooling as well. Generally the fixed electrodes are short lived due to the high temperatures and are frequently replaced.
The fixtures also have various kinds of locators bolted to the fixture body to correctly position the workpieces for welding. The locators potentially provide current paths from the workpieces through the fixture to short the desired weld current path, thereby reducing the effective weld current and lowering the weld quality. The locators and fasteners are provided with various types of electrical insulators to isolate the sheet metal from the fixture body. This results in a multiplication of parts per fixture and causes difficulty in proper fixture design. The use of an electrical insulating material for the fixture is discouraged since electrical insulating materials are typified by low thermal conductivity.